The present invention relates generally to the power regulating and copying arts. More particularly, the invention is concerned with providing a power regulating circuit for controlling the power supplied to fusing apparatus of a xerographic or similar copying machine.
In a xerographic copying machine a resistance heating element is usually employed to fuse the toner image to the supporting copy sheet before the copy is made available to the operator. The apparatus of this invention is intended to regulate the supply of power to the fuser heater to maintain the supply constant irrespective of variations in line voltage or load resistance.
In the process of xerography, for example, as disclosed in Carlson U.S. Pat. No. 2,297,691, issued Oct. 6, 1942, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform, electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them, and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided material such as an electroscopic powder that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be fixed by any suitable means.
Xerography has gained wide commercial success as a convenient and accurate method for the reproduction of copy, producing copy of high resolution. One of the virtues of xerography is its ability to reproduce copy onto a variety of support surfaces that are not sensitized in advance, as is done, for example, in photography. The application of heat to affix xerographic powder images to support surfaces has been extensively employed and typical fusing apparatus for affixing powder images to moving support surfaces is disclosed in Crumrine U.S. Pat. No. 2,852,651.
In the interest of maintaining a consistent degree of fusing fix, it is necessary that the power delivered to the fusing device be maintained at or above some specific minimum value over the tolerance range of line voltage and heater resistance. For certain types of fusing systems, for example, a CHOW fuser, it is possible to perform a worst case design to assure required power, and to allow "on-off" cycling of the fuser lamp via the temperature controller under higher power conditions. However, when the danger exists of exceeding the rating of the assigned power line, or when low inertia (radiant) fusing devices are used, it becomes important to regulate fuser power. This is accomplished by a line power regulator to hold constant the input power to the fuser, automatically compensating for variations both in the line voltage and in the load (the fuser lamp resistance).
Constant power regulators are known in which both the voltage across the load and the current through the load are detected and multiplied in a multiplier circuit to give the total power consumption, and the input power is then regulated up or down to maintain this power consumption constant. The multiplier circuits make such power regulators both complex and costly.
The present circuit, on the other hand, utilizes a summing circuit for summing, rather than multiplying, the load current and load voltage. This provides, in a simpler and cheaper circuit, an approximation of the power consumption which is utilized to control the power input. The percentage accuracy of power consumption with this approximation is quite adequate for a fuser over relatively wide ranges of fuser resistances, and input voltage fluctuations. The summing circuit can be a simple operational amplifier circuit with two commonly connected inputs from, respectively, a voltage tap across the fuser and a tap measuring the current through the fuser. The current input can be the voltge developed across a small resistor in series with the fuser. By selecting the proper resistances of the two input lead resistors, the input voltage contribution from the voltage and current measuring taps can be made equal. This voltage and current control would be in addition to the conventional fuser temperature controls.